1. Field of the Invention
The subject invention relates to a knee joint prosthesis, and particularly a posterior stabilized replacement knee joint prosthesis.
2. Description of the Related Art
A natural knee joint includes the distal end of the femur with articular cartilage, the proximal end of the tibia with articular cartilage and a meniscus between the femur and tibia. The femur and the tibia are held in a proper relationship to the bearing by ligaments. These stabilizing ligaments include the posterior cruciate ligament, the anterior cruciate ligament and collateral ligaments.
Flexion of the knee causes the tibia to rotate relative to the femur about an axis that extends generally in a medial-to-lateral direction and simultaneously causes the contact area of the femur to roll back relative to the tibia. Flexion also generates rotation of the tibia about its own axis. The amount of rotation of the tibia during flexion of the knee is controlled and limited by the ligaments.
The natural knee joint can become damaged or diseased. For example, damage or disease to the knee can deteriorate the articular surfaces of the femur or tibia and can damage the articular cartilage between the bones. The prior art includes prosthetic knee joints to replace a damaged or diseased natural knee. A prosthetic knee joint typically includes a femoral component that is mounted to the distal end of a resected femur, a tibial component mounted to the proximal end of a resected tibia and a bearing between the femoral and tibial components. The inferior face of the femoral component of a prosthetic knee joint typically defines a pair of arcuate convex condyles. The superior face of the bearing has a corresponding pair of arcuately concave regions for articular bearing engagement with the condyles of the femoral component. The superior face of the tibial component may be substantially planar and is disposed in engagement with the inferior face of the bearing.
Prior art prosthetic knee joints have taken many different forms, depending upon the preferences of the orthopedic surgeon, the condition of the natural knee and the health, age and mobility of the patient. Some prior art knee joint prostheses fixedly secure the inferior surface of the bearing to the superior surface of the tibial component. Other prior art knee joint prostheses permit rotational movement between the bearing and the tibial component. Still other prior art knee joint prosthesis permit a controlled amount of anterior-posterior sliding movement between the bearing and a tibial component. Movement of the bearing relative to the tibial component achieves many functional advantages, as described in the prior art. Prior art knee joint prostheses that incorporate certain of the structural and functional features referred to above are disclosed in U.S. Pat. Nos. 4,470,158 and 4,309,778.
As noted above, the inferior bearing surface of the femoral component on most prosthetic knee joints comprises a pair of convexly arcuately condyles. The condyles of the femoral component are in articular bearing engagement with arcuately concave regions on the superior face of the bearing. Thus, the superior face of the bearing typically includes a pair of dished regions each of which has a relatively depressed center portion and a relatively elevated peripheral lip. As explained above, flexion of the knee joint causes the tibia to rotate about a medial-lateral axis relative to the femur. Flexion also causes the tibia to rotate around its own axis. These combined movements cause the condyles of the femur to ride up or climb the concavities on the superior surface of the bearing and to approach the peripheral lips of the bearing. Thus, flexion tends to move the relative components of the prosthetic knee toward dislocation. The degree to which dislocation is possible depends on several factors, most significantly, the presence or absence of ligaments. The likelihood of dislocation also depends upon the degree of flexion and on the degree of congruency between the inferior articular bearing surface of the femoral component and the superior surface of the bearing. For example, climbing of the femoral component on the bearing is not a significant problem in prosthetic knees that have a substantially flat superior surface on the bearing. However, the relatively great incongruency between the inferior bearing surface of the femoral component and the superior surface of the bearing on these prosthetic knees results in a very high contact stress that can damage the bearing. Prosthetic knees that have greater congruency between the femoral component and the bearing provide desirably low contact stress. However, the greater congruency when combined with a bearing that is slidable on the tibial component creates the problem of the tibial component climbing on the bearing, and hence creates the potential of dislocation. Climbing of the femoral component on the bearing also is a particular problem for prosthetic knee joints that employ a posterior stabilization post. In particular, the climbing of the femoral component on the bearing substantially increases sheer forces on the post and can lead to traumatic failure of the prosthesis.
Valgus-varus stability of a knee joint refers to the ability of the joint to resist the lateral forces or rotary forces that would cause rotation of the tibia relative to the femur in the frontal plane. Lateral forces or rotary movements that cause rotation of the tibia relative to the femur in the frontal plane tend to create a dislocation. Such dislocation is particularly likely to occur on either the medial or lateral side of the prosthesis, depending upon the direction of the lateral forces. Such a dislocation in a prior art prosthesis is shown in FIG. 18 hereto.
The prosthetic knee joint is under a compressive loading during normal activities. As a result, valgus-varus moments typically are resisted adequately by the articulating surfaces of the prosthetic components and by the ligaments. However, there are instances where additional valgus-varus stability may be desired, such as those instances where ligaments are deficient.
Some prior art prosthetic knee joints enhance valgus-varus stability by providing a stabilization post that extends into a posterior region between the femoral condyles. This region would be occupied by the posterior cruciate ligament if that ligament were present. Prosthetic knee joints that permit anterior-posterior sliding movement of the bearing on the tibial component provide superior roll back. In this regard, the term xe2x80x9croll backxe2x80x9d refers to a posterior movement of the contact point of the femur relative to the tibia during flexion. Roll back, however, causes the femoral component to climb on the bearing, and thus increases the probability of dislocation. Additionally, this greater roll back and increases of climbing of the femoral component on the bearing substantially reduce shear forces on the posterior stabilizing post for those prosthetic joints that have such a posterior stabilizing post. A prosthetic bearing that can slide posteriorly during flexion avoids impingement between the bearing and anterior soft tissue of the knee. Thus, a prosthetic knee joint with a bearing capable of anterior-posterior sliding movement can avoid discomfort during deep flexion.
A prior art prosthetic knee joint with a stabilizing post and a bearing capable of anterior-posterior sliding movement is shown in U.S. Pat. No. 5,395,401 which issued to Bahler. In particular, U.S. Pat. No. 5,395,401 shows a prosthetic knee having a tibial component and a bearing slidably disposed on the superior face of the tibial component. The inferior surface of the bearing is provided with a dovetailed groove that extends along an anterior-posterior direction and at a location between the two concave condyles formed on the superior surface of the bearing. The bearing shown in U.S. Pat. No. 5,395,401 also includes a notch extending into the posterior portion of the bearing at a location between the two concave condyles of the bearing. The notch registers with the dovetailed groove of the bearing. The prosthesis of U.S. Pat. No. 5,395,401 further includes a control arm with a post that is pivotally engaged in a recess formed on the tibial component. The control arm includes a dovetailed portion that slidably engages in the dovetailed groove on the inferior surface of the bearing. The control arm shown in U.S. Pat. No. 5,395,401 also has a post that extends through the notch in the bearing and between the condyles of the femoral component. The post is dimensioned to slidably engage surfaces of the femoral component between the two convex condyles of the femoral component. However, nothing in U.S. Pat. No. 5,395,401 would prevent dislocation of the femur from the bearing.
Posterior stabilized design concepts developed for fixed bearings are not well adapted to mobile bearing knees. The recessing of the femoral patellar track, common in conventional posterior stabilized devices, is inconsistent with the use of a mobile patellar bearing and has also caused problems with fixed bearing designs. Climb, although a major problem with conforming tibial bearing surfaces, is not usually of great significance with the less conforming fixed bearing articulations.
The normal knee produces about 13 mm of rollback with 10 mm occurring in the first 30xc2x0 of flexion. Typical current posterior stabilized devices fail to produce any significant rollback until about 70xc2x0 of flexion. Thus rollback is not present during stair climbing, which is the most important activity where rollback is needed.
Failure to provide rollback where it is probably the most useful is clearly an undesirable characteristic. Further the highly incongruent cam contact of current posterior stabilized designs appears to be excessive even for limited activities such as arising from a seated position since even one such act can produce permanent deformation of the surface of the plastic cam.
The embodiment described here avoids excessive cam contact, as well as articulating contact, incongruity. It provides much more normal rollback characteristics than earlier posterior stabilized devices. Thus it appears to be a significant advance over these earlier designs.
The subject invention is directed to a knee joint prosthesis with an ability to resist dislocation at high degrees of flexion, but without dislocation resistance at low flexion. The knee joint prosthesis of the subject invention also provides resistance to valgus-varus moments.
The prosthesis of the subject invention includes a tibial component, a femoral component, a bearing and a control arm assembly. The tibial component includes an inferior projection configured for secure mounting in a recess formed in a resected tibia. The tibial component further includes a superior bearing surface having a conical recess extending therein and disposed within portions of the tibial component that define inferior mounting projection.
The femoral component includes a superior surface with a projection for mounting in a recess formed in a resected distal end of a femur. The femoral component further includes an inferior surface defining a pair of convex articular condyles. A notch extends into the posterior end of the femoral component and defines a cam box. The cam box has a pair of substantially parallel spaced apart medial and lateral sidewalls and a femoral cam that extends between superior locations on the sidewalls of the cam box.
The bearing includes a superior surface having a pair of concave arcuate bearing surfaces in articular bearing engagement with the condyles of the femoral component. The bearing further includes an inferior surface disposed in sliding bearing engagement with the superior surface of the tibial component. A dovetail groove is formed in the inferior surface of the bearing, and extends generally in an anterior-posterior direction. The bearing further include a notch extending into the posterior side of the bearing and continuously between the superior and inferior surfaces thereof. The notch is substantially centrally disposed between the medial and lateral extremes of the bearing and registers with the dovetail groove. The anterior end of the notch may include an undercut or step that faces posteriorly and inferiorly. The undercut may engage a portion of the control arm at high degrees of flexion of the joint for resisting dislocation. However, at lower degrees of flexion, the undercut will play substantially no role in the normal operation of the joint. The inferior surface of the bearing may further include a stop recess near anterior portions of the dovetail groove. The stop recess may engage a stop pin on the control arm assembly to limit anterior movement of the bearing.
The control arm assembly includes a conical bearing dimensioned to pivotally engage in the conical recess formed in the tibial component. A dovetail guide extends substantially orthogonally from the superior large diameter end of the conical bearing of the control arm assembly. The dovetail guide is engageable in the dovetail groove formed in the inferior face of the bearing. The control arm further includes a post projecting in a superior direction from the posterior end of the control arm. The post is dimensioned to be received slidably in the notches in the posterior faces of the bearing and the femoral component. Portions of the post adjacent the control arm may define a control arm boss. The boss, if present, is configured to slide into the undercut at the anterior end of the notch in the bearing as the joint approaches maximum deflection. A stop pin may project in a superior direction from the anterior end of the dovetail guide for engagement in the stop recess of the bearing.
The prosthetic joint of the subject invention provides valgus-varus stability in two ways. Under loading conditions the normal compressive load will press the femoral condyles against the matching superior bearing surface of the bearing. The match is such that under compression any rotation of the femoral component occurs around an axis extending in an anterior-posterior direction. Rotation about such an axis produces impingement between the side surfaces of the post of the control arm and the sidewalls of the cam box. This contact produces a reaction force that resists any valgus-varus moment applied to the joint. During non-load bearing phases, where any valgus-varus moment is small, the post may be subject to small bending loads since joint compression will not exist under these conditions. However, the post can be made strong enough to resist such bending moments.
As flexion of the joint progresses, the box cam surface will engage the cam surface of the post. This engagement will commence at about 30xc2x0-35xc2x0 flexion. Flexion beyond about 30xc2x0-35xc2x0 will force the femoral component posteriorly. Compressive force on the bearing and its concave shape will cause the bearing to move with the femoral component. This posterior movement, or femoral rollback, improves quadricep effectiveness. Slightly beyond about 120xc2x0 of flexion, it is desirable to prevent any additional posterior motion of the bearing. This can be accomplished by engagement between the post stop surface of the control arm and the recess stop surface defined by the undercut in the notch of the bearing.